A typical radio access wireless (RAN) network includes a number of base stations, access points, access nodes, or the like, that radiate to define wireless coverage areas, such as cells and cell sectors, in which wireless communication devices (WCDs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a WCD within coverage of the RAN may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other WCDs served by the RAN.
In general, a RAN may operate in accordance with a particular radio access technology or “air interface protocol,” with communications from the base stations to WCDs defining a downlink or forward link and communications from the WCDs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE) or Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communications (GSM), in addition to various Wireless Local Area Network protocols such as the IEEE 802.11 (WiFi) protocols, among others. Each protocol may define its own procedures for registration of WCDs, initiation of communications, handover between coverage areas, and functions related to air interface communication.
RANs such as these may provide connectivity with a communication services platform that facilitates advanced communication services, such as voice over Internet Protocol (VoIP) based call connections and other packet-based real-time media services (e.g., video sessions, etc.) for instance. When a WCD is served by a particular RAN, the WCD may then engage in communication via the RAN with the communication services platform and, via the communication services platform or associated equipment, with various remote entities such as remote call parties. An example of such a platform is an Internet Multimedia System (IMS), which is arranged to facilitate packet-based communications of various forms and which may support WCD access through various RANs or landline access networks. Typically, such an IMS would support industry standard packet-based signaling and bearer communication protocols, such as Session Initiation Protocol (SIP) signaling and Real-time Transport Protocol (RTP) bearer communication for instance.
To facilitate IMS service, once a WCD is registered to be served by a particular RAN, the WCD may then register with the IMS, via the RAN, and/or the RAN may register with the IMS on behalf of the WCD. For instance, if the WCD has an assigned IP address to facilitate wireless packet-data communication, the WCD may then engage in SIP registration signaling with the IMS via the RAN, or the RAN may engage in SIP registration signaling with the IMS on behalf of the WCD. Such registration may trigger authentication of the WCD, reservation of resources in the IMS to serve the WCD, and notification of the IMS where the WCD can be reached for incoming communications, among other functions.
In turn, the WCD and/or the RAN may engage in further signaling with the IMS to set up communications such as VoIP calls or the like between the WCD and remote parties. For instance, if the WCD has an assigned IP address, the WCD may engage in SIP signaling with the IMS via the RAN to place or receive a VoIP call, which may result in setup of a call leg between the IMS and the WCD via the RAN, another call leg between the IMS and a remote party, and an IMS gateway or other node bridging those two call legs together to facilitate communication between the WCD and the remote party. Alternatively, the WCD may engage in signaling communication with its serving RAN to set up a call, and a node of the RAN may in turn engage in signaling communication with the IMS to set up the call via the IMS. Other arrangements are possible as well.
An IMS may also support handover of a communication when a WCD transitions from being served by one RAN to being served by another RAN. For instance, if the WCD has an IP address for packet-data communication via the new RAN, the WCD may engage in SIP signaling with the IMS via the new RAN to re-register via the new RAN and to cause the IMS to establish a replacement call leg with the WCD via the new RAN. Alternatively, if the WCD does not have an IP address for packet-data communication via the new RAN, the WCD may engage in signaling with the new RAN, and the new RAN may then engage in signaling with the IMS to establish a replacement call leg with the WCD via the new RAN.
To facilitate such handover, the IMS may supply the WCD in advance with an IMS transfer-address at the time the WCD registers to be served by the IMS. Such an IMS transfer-address may point to or otherwise identify a node in the IMS that would control communications for the WCD. Thus, when the WCD is engaged in a communication served by the IMS and the WCD transitions to be served by a new RAN, the WCD can send that IMS transfer-address via the new RAN in SIP signaling to the IMS, or the WCD can send that IMS transfer-address to the new RAN and the new RAN can send the IMS transfer-address to the IMS, so that the IMS node that controls communications for the WCD can appropriately extend the ongoing communication to the WCD via the new RAN.